Dental investment composition



I the low heat 0 mixture of this type may not be Patented Mar. 14, 1933 umrao STATES PATENT OFFICE O. DAILEY, OF HAYWOOD, ILLINOIS, ASSIGNOB '10 UNITED sm'rns' ensuu COMPANY, OF CIIIOAGO, ILLINOIS, A CORPORATION OF ILLINOIS DENTAL INVESTMENT COMPOSITION No Drawing. Application filed July 8, 1980,

This invention relates to compositions of matter and has reference more particularly to compositions suitable for use-in mechanical dentistry.

In the casting of partial dentures, it is common practice to make an impression of the mouth with a composition commonly own as adisso1ving impression plaster. Using the impression cast as a mold, a model is cast from an investment compound which ordinarily consists of a mixture of plaster of Paris and fine silica in approximately the ratio of 30 parts silica to 70 parts plaster of Paris. Small amounts of other ingredients, such as boric acid or graphite, are sometimes added to give certain desirable efiects. the model has set, both model and mold containing same are placed in boiling Water until the mold is entirely disintegrated. The model remains intact and represents accurately the exact shape of the mouth to which the denture is to be fitted. On this model is built up in Wax a pattern of the denturewhich will eventually be installed in the mouth. The model with wax pattern is then placed in a metal flask and an investment compound in plastic state is model, this composition eing usually the same as employed for the model. After this investment has set, the wax is boiled out and the whole is heated to a temperature ranging from 300 to 600 F, for a period of 12 to 16 hours in order to thoroughly calcine the plaster and remove all traces of wax. The flask containing the investment is then placed in a gold casting machine and molten gold or other metal or alloy is cast into the depression left by the wax. After cooling, the plaster is scraped away and the denture is cleaned and polished ready for insertion in the mouth.

The technique described above is known as technique of preparing partial dentures. It is necessary to use a rather low proportion of silica with respect to plaster in order that the model may be immersed in boiling water for a considerable length of time and still retain suiiicient strength and hardness to prevent its disintegration. A burned to a oured around the After h semi No. 465,948. Renewed July so, 1932.

siderable contraction. It is desirable that investment compounds of this type ossess the property of expanding from 0. -,-1.0% on bemg heated to the maximum temperature and calcinof the gold from 1ts melting point temperature to mouth temperature. Thermal expansion of investment compounds is increased by increasing the amount of silica present. Also, the use of a high silica ratio enables us to burn out the investment at a lgher temperature than is possible with a low silica ratio without warping and crackmg of the mold. A high silica content is therefore very desirable in order to secure the necessary thermal expansion and other desir-. able characteristics described. Heretofore it has been impossible to use a high silica 10w plaster formula for this type of work, hecause of the fact that formulas of this type will tend to disintegrate in boiling water when the dissolving impression plaster is re moved, thus leaving a soft and crumbly surface which may be untrue to dimension and which will result in final rough and ill-fitting dentures. a

An object of this invention therefore is to produce an investment composition which has ahigh thermal expansion.

Another object of the invention istopro- -ment compositions in other respects herein-- after specified and claimed. p

In the copending application of Randel and Dailey, Serial No. 384,343, filed August 9, 1929, and entitled High strength calcined gypsum, a new high strength calcined gypsum is \described, together with its method of manufacture. This high strength calcined gypsum has unique properties in that its compressive and tensile strength equal or, exceed that of Portland cement, whereas its setting time is a proximately that of ordinary plaster of aris, viz., 15 to 30 minutes. The compressive strength of this roduct will range from 2500 to 5750 poun s per s uare inch and it has a pouring consistency 0 less than 50%. This high strength calcined gypsum which I shall in the future designate as alpha gypsum, is manufactured by treating lumps of gypsum rock having a diameter of to 2" in a closed container with steam at 1 to pounds gage pressure. Steam at 15 to 50 pounds pressure may be used but outside of the preferred range of 17-20 pounds gage, the strength and quality of the resultin product diminishes. calcination is carried on for 5 to 7 hours with a constant steam pressure and with a constant withdrawal of water of condensation obtained from the heating steam and also from 26 the expelled water of crystallization. After calcination, the product is dried while maintaining at a temperature under 400 F. and the product is ground so that 85% or more of same passes through a 100 mesh screen. 30 The crystals of ordinary first settle calcined stucco as seen under crossed nicols in a petrogra hic microscope, are very fine and needle-1i e, these crystals being mostly less than 5 microns in diameter. The crystals of alpha gypsum when examined at the same magnification under the same microscope, appear rather short, thick and well formed. The crystals of alpha gypsum are practically pure crystalline calcium sulphate hemiydrate, and when examined under this microscro e, they appear brilliantly colored, while litt e or no color is visible in the case of the. tiny needle-like crystals of ordinary plaster of Paris. The unique properties of our improved alpha gypsum, together with another cooperating gum mgredient, has enabled me to prepare an investment composition which has many marked advantages.

The preferred formula is as follows:

69 Alpha gypsum 40 Powdered silica Gum arabic Water about 33 cc. per 100 grams of dry mix.

Sufficient retarder or accelerator is used to give a product having the desired time of set. Small amounts of other materials may be added, if desired, to give greater thermal expansion or improve other characteristics of no the compound. Water is added to the mixture to bfing to molding consistency and this combines with the calcined gypsum to form hydra-ted gypsum.

The alpha ypsum in the above formula 65 may be varietf from 30 to gms., the pow- This steam dered silica from 30 to 7 0' gms. and the gum arabic from A to 2 gms. with varying results. Tapioca dextrine, and other hardening agents may be used instead of the gum arabic and powdered flint, quartz or other siliceous fillers mayvple useil instead of the en a p powdered silica. ha gy sum is used, the gum arabic becomes a desirable but non-essential ingredient, and when g'um arabic is used, the alpha gypsum becomes a desirable but non-essential ingredient and other forms of calcined gypsum, such as plaster of Paris, may be substituted. In some cases the retarder or accelerator may be omitted. Common accelerators known to the art are finely ground gypsum, potassium sulphate, alum, etc. Commonly used retarders are hydrolized' protein matter, known as commercial retarder and such chemicals as soluble citrates, phosphates and acetates. The alpha gypsum is preferably ballmilled but may be ground otherwise to pass a 100-150 mesh screen.

My improved investment composition may be burned at a high temperature, say 900 to 1600 F. and the wax is completely burned out in a period of thirty minutes to one hour as contrasted with the burning time required in other compositions of 12 to 16 hours. Because of the high silica content, the composition has a high thermal expansion of about 0.75 percent at casting temperatures, with the result that it is possible to obtain proper dimensions of the finished denture. The cast composition may be immersed for long periods of time in boiling water and still retain a hard surface true to dimensions. This latter propert of resisting boiling water is imparted to t e composition especially by the gum arabic. This gum arabic in the mix imparts to the calcined gypsum the property of great hardness and of resistance to loss of strength upon wetting.

V I would state in conclusion that while the illustrated examples constitute a practical embodiment of my invention, I do not wish to limit myself precisely to these details, since manifestly, the same may be considerably varied without departing from the spirit of the invention as defined in the appended claims.

Having thus described my invention, I

claim as new and desire to secure by Letters Patent:

1. An investment composition comprising a mixture of the followingingredients:

Grams Alpha gy sum 40 Powder silica 60 Gum arabic 2. An investment composition comprising alpha gypsum, a powdered siliceous materia and gum arabic.

3. A composition of matter comprising alpha. gypsum, a siliceous filling material, and a hardening agent.

4. An vestment com osition comprising alpha gypsum, tapioca extrine, and a sillceous filling material.

5. A composition of matter comprising alpha gypsum, powdered silica, gum arabic, retarder, and boric acid.

6. An investment composition comprising alpha gypsum 30-70 grams, siliceous filling material 30-70 grams, gum arabic 2 grams and retarder or accelerator sufficient to bring to the desired set with Water.

7 An investment composition comprising alpha gypsum, a siliceous fillin material, gum arabic and commercial retar er.

MANVEL C. BAILEY. 

